The present invention relates to tracking control for a magnetic recording/playback apparatus (which will be termed simply "VTR" hereinafter) and, particularly, to a method of tracking control for realizing variable-speed playback at arbitrary playback tape speeds in a VTR using four kinds of tracking control pilot signals and an electromechanical transducer for moving the head.
A method of noise-free variable-speed playback using an electromechanical transducer such as a piezoelectric element has been realized in VTRs of the VHS system. In this method, the capstan FG (frequency generator) signal, which virtually represents the tape speed is counted to produce a sawtooth wave. The difference between this sawtooth wave and the sawtooth wave necessary for still picture reproduction is used to produce an actual preset voltage waveform to be supplied to the electromechanical transducer. The capstan FG signal does not indicate the exact tape speed due to slippage of the tape between the capstan and pinch roller, and therefore the preset voltage waveform is corrected by being reset by a playback control signal.
Although the above-mentioned method is effective for VTRs having the control signal, it cannot be applied directly to VTRs using tracking control pilot signals (which will be termed simply "pilot signals" hereinafter) instead of the control signal. The reason is that there is no control signal to be reset and that pilot signals, when four kind of signals are used, recorded on the scanning track need to be identified, as will be described in detail later, which necessitates the introduction of a new method.
Next, the four-frequency pilot system will be summarized, and a conceivable variable-speed playback system and its problems will be described.
FIG. 1 shows magnetic recording traces of the pilot signals. In the figure, A.sub.1, B.sub.1, A.sub.2, and so on are magnetized traces produced by head A and head B, and each recording track has a record of a video signal of one-field length. The pilot signals indicated by f.sub.1 through f.sub.4 are recorded over the video signal and arranged orderly for each field. The pilot signals have frequencies as listed in Table 1, in which f.sub.H denotes the frequency of the horizontal sync signal.
TABLE 1 ______________________________________ f.sub.1 = 102.5 (kHz) .apprxeq. 6.5 f.sub.H f.sub.2 = 118.9 (kHz) .apprxeq. 7.5 f.sub.H f.sub.3 = 165.2 (kHz) .apprxeq. 10.5 f.sub.H f.sub.4 = 148.7 (kHz) .apprxeq. 9.5 f.sub.H ______________________________________
Pilot signals of adjacent tracks have a difference of frequencies of f.sub.H or 3f.sub.H as shown in FIG. 1. Accordingly, by extracting each frequency difference and comparing the levels, a tracking error signal related to the tracking deviation can be obtained.
FIG. 2 is a block diagram of a circuit for producing the tracking error signal. In the figure, a terminal 1 receives the reproduced RF signal, and a lowpass filter 2 extracts only pilot signals. A balanced modulation (BM) circuit 3 produces the difference of frequencies of the pilot signal received and a reference signal received at terminal 4. For example, the pilot signal reproduced when the head scans the track A.sub.2 in FIG. 1 includes f.sub.3 which is reproduced on the main scanning track and a composite signal of f.sub.2 and f.sub.4 which is reproduced as a crosstalk signal. The reference signal at this time is the pilot signal f.sub.3 recorded on the main scanning track. The BM circuit 3 at this time provides outputs having frequency differences between f.sub.3 and each of f.sub.2, f.sub.3 and f.sub.4, and they include signals of f.sub.H and 3f.sub.H. These differential frequency signals are picked up by a tuning circuit 5 which extracts f.sub.H and a tuning circuit 6 which extracts 3f.sub.H , and fed through detecting circuits 7 and 8, respectively, to a comparison circuit 9, which is followed by an analog inverting circuit 10 and an electronic analog switch 11. The switch 11 operates in response to the head switching signal received at a terminal 12, and the output of the comparison circuit 9 and its inverted version are outputted at a terminal 13 alternately for every field. The reason for the need of signal inversion is that the extracted tracking signal has opposite polarities for head A and head B. Namely, a deviation of head A to the right causes an increase in the f.sub.H component, while that of head B causes a decrease in the f.sub.H component. In consequence, a tracking error signal whose polarity is independent of the playback heads can be obtained at the terminal 13.
In order for the tracking control system based on the foregoing principle to accomplish variable-speed playback without producing noise on the screen, the following requirements must be fulfilled.
(1) The scanning start point of the head must be located virtually at the center of the intended track.
(2) The angle of the head scanning trace must coincide with the angle of the recording track.
(3) The pilot signal recorded on the main scanning track must be selected for use as the reference signal supplied to the BM circuit.
Of the three requirements, items (1) and (2) are also applied to the conventional control system using the control signal, while item (3) is applied only to methods using four kinds of pilot signals.
In the normal-speed playback operation in which the tape speed is the same as recording, the reference signal supplied to the BM circuit is switched for each field in the order of f.sub.1 .fwdarw.f.sub.2 .fwdarw.f.sub.3 .fwdarw.f.sub.4 .fwdarw.f.sub.1. The reason is that the pilot signal reproduced on the recording track also has the order of f.sub.1 .fwdarw.f.sub.2 .fwdarw.f.sub.3 .fwdarw.f.sub.4, and even if it is not coincident with the reference signal initially, the resultant tracking error signal acts to control the phase of tape control so that tracking control will settle at the time point when both signals coincide with each other.
When a record is played at a tape speed different from the recording tape speed, the order of switching the reference signal varies depending on the playback tape speed. For example, in the double-speed playback mode, the reference signal is switched in the order of f.sub.1 .fwdarw.f.sub.3 .fwdarw.f.sub.1 .fwdarw.f.sub.3, while in the triple-speed playback mode, the order is f.sub.1 .fwdarw.f.sub.4 .fwdarw.f.sub.3 .fwdarw.f.sub.2 .fwdarw.f.sub.1.
When the variable-speed playback mode includes a smaller number of tape speeds, e.g., only forward triple-speed playback, the system may be designed such that the order of reference signal switching in that mode is preset in the memory circuit and the reference signal is produced in accordance with the command retrieved from the memory circuit. This method may be effective for the case of a fast playback mode having a limited number of tape speeds, however, it is not convenient for the case of many playback speeds because of a great capacity needed for the memory circuit for storing the order of reference signal switching in correspondence to all tape speeds.
Another problem is that when one playback speed is changed to another speed, the tape feed might not follow a changed reference signal satisfactorily. If the new playback tape speed is greatly different from the current tape speed, the tape feed cannot follow the command for a significant time length, resulting in the creation of noise in the reproduced picture. This drawback may be avoided by sensing the tape speed at every moment and generating the piezo-electric element application voltage accordingly; however it needs complex processing. Alternatively, the tape feed response performance is examined in advance and the piezo-electric element application voltage is generated to suit the performance. However, this method also needs complex processing and the tape feed response may possibly change due to a change in the load on the tape feed mechanism caused by an environmental change, aging of the mechanism, or the like.
For a playback system operable for continuous variable-speed playback, a d.c. component included in the voltages applied to the piezo-electric element and voice coil often adversely affects the life and performance of the devices and reduces the power efficiency of the whole system including the driving stage. Therefore, application of a d.c. component to the system component devices must be avoided throughly.